Juventus have been left sweating on the fitness of star man Cristiano Ronaldo, with the Portuguese set to undergo tests on an ankle injury.

The former Real Madrid man picked up a knock to his left ankle during the Bianconeri's 1-0 win over Bologna on Sunday.

But concerns have grown since then, with Ronaldo unable to participate in full training with the rest of the Juve squad on Wednesday, and will now be further examined by the Serie A champions' medical team.

Fellow forward Douglas Costa also trained separately on Wednesday, with a club statement reading: "Both Douglas Costa and Cristiano Ronaldo trained seperately from the group.

"For CR7, check-ups have been scheduled for today and tomorrow following a knock to his left ankle during the game against Bologna."

The injury could not come at a worse time for Massimiliano Allegri's side as they enter a crucial period in their season.

Though the Scudetto looks almost assured with Juve having opened up a 13-point lead at the top of the Serie A table, Sunday's match with second-placed Napoli remains a tough test.

It could even be argued that a visit to the San Paolo Stadium will provide one of the last big hurdles in Allegri's pursuit of an unbeaten domestic campaign.

Moving forward, the return leg of the Champions League last-16 tie with Atletico Madrid may prove to be defining when looking back on Ronaldo's first season at the Allianz Stadium.

Diego Simeone's side hold a 2-0 lead from the first leg in Madrid, and Juventus will be desperate to have a fighting fit Ronaldo available as they look to turn the tie on its head in Turin.

Ronaldo has enjoyed a fine debut campaign in Italy, scoring 21 goals in 34 appearances across all competitions, with 19 of those strikes coming in Serie A.

His form in the Champions League has been less stellar, with the competition's highest-ever goalscorer having only found the net on one occasion in continental competition thus far in 2018-19.